sysctl/sched: Fix 'defined but not used' warning

sysctl/sched: Fix 'defined but not used' warning

Since commit ("sched/numa: Implement NUMA home-node selection code")
building a kernel with CONFIG_SMP disabled causes the following
warning:

  kernel/sysctl.c:259:12: warning: 'min_sched_tunable_scaling' defined but not used [-Wunused-variable]
  kernel/sysctl.c:260:12: warning: 'max_sched_tunable_scaling' defined but not used [-Wunused-variable]

Reported-by: Fabio Estevam <fabio.estevam@freescale.com>
[ Ingo preferred extra #ifdef variant over the __maybe_unused ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-p9w5w57ylinrj9zakvhc5zay@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>